1. Field of the Invention
The present invention relates to a driver circuit of a display device. More specifically, the present invention relates to a driver circuit of a liquid crystal display (hereinafter, referred to as xe2x80x9cLCDxe2x80x9d) using thin film transistors (hereinafter, referred to as xe2x80x9cTFTsxe2x80x9d) for making up a scan driver circuit and an image signal driver circuit.
2. Description of the Related Art
Recently, a new TFT has been under development for use as a driver element or a pixel driver element of an active matrix type LCD, and such a TFT utilizes an active layer made of polysilicon film that is obtained by irradiating a laser beam to an amorphous silicon film.
Especially, since each of the TFTs of the driver circuit needs high electron mobility, the polysilicon having higher electron mobility than the amorphous silicon is useful as the active layer.
A TFT constituting the conventional driver circuit will be explained as follows.
FIG. 1 is a block diagram showing a conventional LCD, and FIG. 2 is a plan view showing a relationship between the position of the TFT of each driver circuit and irradiation of a laser beam. In FIG. 2, each channel of the driver circuits 102 and 105 is illustrated in a hatched box.
The driver circuit of the LCD includes a LCD panel 100, a scan signal driver circuit 102 and an image signal driver circuit 105, which are disposed on the LCD panel 100. The scan signal driver circuit 102 supplies a scan signal to the display pixel portion 108 including a plurality of display pixels 101. The image signal driver circuit 105 supplies a display signal to the display pixel portion 108. The driver circuits 102 and 105 are respectively disposed in horizontal and vertical directions of the display pixel portion 108 in the LCD, and circuits 102 and 105 are disposed in directions perpendicular to each other with respect to the display pixel portion 108 of the LCD.
The scan signal driver circuit 102 includes a shift register 103 and buffer TFTs 104 for supplying the signal from the shift register 103 to the display pixels 108. Each of the shift register 103 and the buffer TFTs 104 is made of the TFT.
The image signal driver circuit 105 includes a shift register 106 and sampling TFTs 107, each of which is constituted by the TFT.
The active layer of the above-mentioned TFT is made of polysilicon. This polysilicon is made by irradiating a laser beam 109 to an amorphous silicon film formed on a substrate. The laser beam 109, e.g., a belt-shaped excimer laser having a dimension of 500 xcexcmxc3x97150 mm, is scanned in the direction for example from the left end to the right end of the LCD panel (in the direction of the arrow 12 in FIG. 2) by the feeding pitch of 35 xcexcm.
However, directions of the channel width W of the TFTs 102 and 107 making up the driver circuits are different from each other.
In other words, as shown in FIG. 2, the direction of the channel width W in the scan signal driver circuit 102 is the same as the scanning direction of the laser beam shown in the arrow 12. On the other hand, the direction of the channel width W in the image signal driver circuit 105 is perpendicular to the scanning direction of the laser beam shown in the arrow 12.
If the channel width W of each TFT is 300 xcexcm, the channel length L is 7 xcexcm, and the feeding pitch of the laser is 35 xcexcm, which means that the laser is scanned eight or nine times for the scan signal driver circuit 102, while only once for the image signal driver circuit 105 in direction of the channel length L of the circuit 105 since the laser is scanned in the direction of the channel length L.
Therefore, if the energy of the laser irradiated to each spot varies, the crystal size of the polysilicon formed by the irradiation may vary. In this case, characteristics of the TFT, e.g., the electron mobility, may vary.
As a result, the image signal supplied from the sampling TFT of the image signal driver circuit 105 to the display pixel portion may be not uniform among the signal lines, and unevenness may show up on the display. This is the problem to be solved.
An object of the present invention is to solve the above-mentioned problem by providing a driver circuit of the display device, which can adjust the channel width direction of the TFT of the scan signal and image signal driver circuits so that each channel of the TFTS of the driver circuits is irradiated by the laser plural times for a uniform display.
In the present invention, the channel width direction of a buffer thin film transistor of a scan signal driver circuit for supplying a scan signal to a display pixel is identical with the channel width direction of a sampling thin film transistor of an image signal driver circuit for supplying an image signal to the display pixel.
In addition, the channel width directions of the buffer thin film transistor of the scan signal driver circuit for supplying the scan signal to the display pixel and the sampling thin film transistor of the image signal driver circuit for supplying the image signal to the display pixel are identical with a scanning direction of a laser beam irradiated to an active layer of the thin film transistors.
In another aspect, the present invention provides a driver circuit of a display device having display pixels arranged in a matrix on a substrate. A scan signal driver circuit for supplying a scan signal to the display pixels extends along one of adjacent sides on the peripheral area of the substrate. An image signal driver circuit for supplying an image signal to the display pixels extends along the other side of the adjacent sides on the peripheral area of the substrate. The channel width direction of the buffer thin film transistor of the scan signal driver circuit is identical with the channel width direction of the sampling thin film transistor of the image signal driver circuit.
In another aspect of the present invention, the channel width directions of the buffer thin film transistor of the scan signal driver circuit and the sampling thin film transistor of the image signal driver circuit are identical with the scanning direction of the laser beam irradiated to the active layer of the thin film transistor in the above-mentioned driver circuit.
In addition, a plurality of the sampling thin film transistors whose channel width direction is identical with the extending direction of the image signal driver circuit is arranged in parallel in the direction away from the display pixels to make up one unit. A plurality of the units are arranged in the extending direction of the image signal driver circuit.
Furthermore, a plurality of the sampling thin film transistors whose channel width direction is identical with the extending direction of the scan signal driver circuit are arranged in parallel in the direction away from the display pixels to make up one unit. A plurality of the units are arranged in the extending direction of the scan signal driver circuit.
Furthermore, the channel width directions of the thin film transistors making up a scan signal side shift register and an image signal side shift register are identical with the scanning direction of a laser beam irradiated to an active layer of the thin film transistors.
In another aspect, the present invention provides a driver circuit of the display device that is a liquid crystal display device performing display by driving a liquid crystal disposed between a pair of substrates. On one of the substrates, pixel electrodes constituting the display pixels, display thin film transistors performing in accordance with the scan signal to supply the image signal to the pixel electrodes, a scan signal driver circuit for supplying the scan signal to the pixel electrodes, and an image signal driver circuit for supplying the image signal to the pixel electrodes are formed. An active layer including the channels of the display thin film transistors, the buffer thin film transistors of the scan signal driver circuit and the sampling thin film transistors of the image signal driver circuit is made of a polycrystalline film formed by irradiating the laser beam to an amorphous semiconductor film while scanning in a predetermined direction.
In another aspect of the present invention, the channel width direction of the buffer thin film transistor of the scan signal driver circuit and the channel width direction of the sampling thin film transistor of the image signal driver circuit are identical with the extending direction of the image signal driver circuit on the substrate in the above-mentioned driver circuit.
According to the above-mentioned driver circuit of the display device, TFTs of the driver circuit having uniform characteristics without influence of variation of energy of an irradiated laser can be obtained. As a result, each of the TFTs can supply uniform signals to a display pixel portion for a uniform display.
In addition, as mentioned above, a plurality of thin film transistors (sampling or buffer thin film transistors) are arranged in the direction away from the display pixels so as to make up one unit, and a plurality of the units are arranged in the extending direction of the driver circuit. With this layout, the channel width directions of the two driver circuits are identical with each other, so that the increase of the area for the driver circuit can be minimized.